The present invention is directed, in general, to wireless communications and, more specifically, to addressing multipath fading in wireless communications through incremental redundancy improving packet reception probability in wireless systems employing turbo-coding.
In wireless channels, where signals can arrive at different times following different paths, the received signals can experience large amplitude and phase variations due to the interference of the different paths. This phenomena is referred to as multipath fading, and the effect is a critical parameter for consideration in receiver design. For terrestrial mobile telephony, multipath fading may dictate the entire system capacity and throughput rate.
Wireless signals transmitted from or to a mobile station may be reflected from the terrain, fixed or mobile objects in the propagation path such as buildings or vehicles, or from a discontinuity in the atmosphere. If the energy of the reflected wireless signal is not significantly absorbed and/or attenuated, a plurality of different propagation paths for the wireless signals are created between the transmitter and receiver, referred to as multipath propagation, which allows the wireless signals to xe2x80x9cbendxe2x80x9d around corners and propagate beyond terrain features and objects obstructing the line-of-sight between the base and remote stations.
Three problems associated with multipath propagation for remote stations include (1) the delay spread of the received signal, (2) the Rayleigh fading in received signal strength caused by varying phase shifts between different paths, and (3) the varying frequency modulation due to the Doppler shift between various propagation paths. The fact that propagation paths for reflected signals are longer than the direct propagation path from the transmitter to the receiver (e.g., from the base station to the remote station) gives rise to signal delays and, because various paths lead to slightly different arrival times, the received signal xe2x80x9cspreads.xe2x80x9d Rayleigh fading results from differences between the phase and amplitude of the reflected wireless signals relative to the phase of a directly propagating signal, attenuating the signal strength at the receiving end (e.g., reception of two signals propagated along two different paths and arriving with a phase difference of 180 degrees results in cancellation in the receiver). Doppler shift is caused by the movement of the remote stationxe2x80x94or a vehicle or other reflecting objectxe2x80x94in relation to the base station, such that the mean frequencies of both the received reflected signal and of the directly propagated signal deviate from the mean frequency of the transmitted signal by a different amount and in a different direction.
In general, multipath fading causes wide variations in received signal amplitudes, and much effort has been expended in attempting to mitigate the impact of multipath fading. One suggestion for minimizing the effects of multipath fading, set forth in the High Data Rate (HDR) standard, involves transmitting redundant turbo-coded packets with new parity bits when an error occurs during decoding of the original packet at the receiver. Turbo-coding in communications systems involves coding/decoding information in stages in order to avoid retransmission of a full L-bit packet upon occurrence of a packet error. In addition to a set of code bits generated by an encoder using a turbo-coding scheme, a punctured set of code bits is typically generated and stored in transmitter memory. The original set of code bits is transmitted as an L-bit data packet to a receiver, which stores received data samples corresponding to the original set of code bits. The receiver decodes the data packet using a turbo-decoder and determines whether an error occurred in decoding the received data packet. If so, the received data samples are maintained in memory, and a request for more information is made. Some or all of the punctured information is then forwarded from the transmitter to the receiver. A second stage of turbo-decoding combines the new data samples with the stored original received data samples such that there is a high likelihood that decoding is correct at this point, but additional stages of decoding may be used.
In order to take advantage of the redundant packet transmission upon occurrence of decoding errors relating to turbo-coded packets, the remote receiver design must implement a suitable method for combining the decoded original packet or set of code bits and the second (punctured) packet or set of code bits. There is therefore a need in the art to improve decoding performance in incremental redundancy communications systems utilizing turbo-coding.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in an incremental redundancy wireless communications system, a mechanism for employing bit probability estimates generated by a turbo decoder to weight the bits of a received packet before combining the received packet with a redundant packet. Bits are individually weighted by the corresponding bit probability estimate and all bits within the received packet are weighted by a block confidence estimate generated from the cumulative bit probability estimates for the received packet. Weighted packet bits are combined within either weighted packet bits for the redundant packet or unweighted received bits for the redundant packet. The bits are combined by adding counterpart weighted (or weighted and unweighted) systematic and parity bit values within the two packets while inserting disjoint parity bit values within the combined result. Decoding performance for incremental redundancy systems utilizing turbo decoding is improved.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.